Devices for mixing liquid reaction components, which are used in the production of in-mold foamed objects, in particular those formed of a polyurethane are known in the art. These devices generally comprise a storage container for each reaction component, with one feed pipe leading from each storage container to a common mixing head where the pipe terminates in a pressure controlled injection jet, which points into a mixing chamber. In each case a return pipe containing a shut-off valve branches off from the pressure side section of the feed pipe, and opens out into the appropriate suction side section of the feed pipe.
The pipes of this type of device "breathe"; that is to say they expand under the pressure build-up of the reaction components. Since the reaction components contain gases and air in the form of small bubbles, such components have a certain compressibility, which has an effect on the opening intervals of the injection jets. If, because of the differences in compressibility of the components, one of the reaction components passes prematurely into the mixing chamber a part of the premature reaction component will pass unmixed through the exit opening of the mixing chamber into the interior of the mold. This creates undesirable defects in or on the foamed object. In spite of the problems inherent in the presence of these gas bubbles, the presence of gas or air in the reaction components is necessary, because upon expansion in the mixing chamber, the small bubbles form the seeds for the beginning of the foaming reaction.
An object of the present invention is therefore to provide a device which ensures the simultaneous entry of the reaction components, so that foam objects can be produced without defects.